This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7 119 from an application entitled Cathode Material for Electron Beam Device earlier filed in the Korean Industrial Property Office on Jan. 10, 2000, and there duly assigned Serial No. 963/2000 by that Office.
1. Field of the Invention
The present invention relates to a cathode material for an electron beam apparatus, and more particularly, to a cathode material used as an electron emission source of a vacuum electron beam apparatus such as a cathode-ray tube.
2. Description of the Background Art
Cathode systems in use today are based mainly on electron emission systems where electrons are emitted by an oxide cathode heated indirectly by a filament. However, these systems have difficulty in emitting more than 1 A/cm2 (amperes per square centimeter) of current density, due to a limitation of electron emission power.
Also, the oxide cathode is fragile and has low adhesive strength to metal materials loaded, and thus the cathode apparatus with this type of the cathode has a short lifetime. For example, even if only one of the three oxide cathodes of the color Braun tube is damaged, the total apparatus, which is costly, will be out of order.
Owing to these reasons, there have been active attempts to apply to the cathode-ray apparatus highly efficient metal cathodes that are free of the disadvantages of the oxide cathodes described above.
For instance, a metal cathode based on lanthanum hexaboride(LaB6) is known to have a higher degree of strength and electron emission power compared to oxide cathodes, and a single crystal cathode can emit a higher electron current density on the order of 10 A/cm2. However, the lanthanum hexaboride cathode has a short lifetime, and thus it has been used only partially in a vacuum electronic apparatus whose cathode unit can be replaced. The reason that the lanthanum hexaboride cathode has a short lifetime is due to high reactivity with the components of a heater, and to the fact that lanthanum hexaboride is in contact with the components of the heater, e.g. tungsten, to form fragile compounds.
U.S. Pat. No. 4,137,476, issued to Ishii, et al., for Thermionic Cathode discloses a cathode where a barrier between lanthanum hexaboride and the body of the heater is formed, in order to eliminate the reaction possibility. But, according to this method, the production cost of the cathode increases significantly and it is difficult to improve the lifetime of the cathode.
Also, as a material with a high electron emission specific density, an alloy including iridium and a small amount of a rare earth metal of the cerium group lanthanum, cerium, praseodymium, neodymium, samarium), (S. E. Rozhkov et. al, Work function of the alloy of Iridium with Lanthanium, Cerium, Praseodymium, Neodymium, Samarium, Journ. Radiotechnika I electronica, 1969, v.14, No.5, p936-analogue) has been known.
However, this alloy has the property that the speed of the active components to migrate to the cathode surface decreases with operation of the cathode, so that as time goes by, the work function increases rapidly and the electron emission property and the resistance of the cathode to ion impact decrease. The binary alloy is fragile and thus the cathode unit is not easy to manufacture and not operable at high temperature due to its low melting point. Therefore the alloy is not suitable for applying to an electronic apparatus requiring a long lifetime and operation stability.
SU (Soviet Union) Patent No. 616662 discloses a cathode material of a trinary alloy of iridium, cerium and hafnium. The cathode material has excellent emission stability and plasticity, but its melting point is low and thus it is not applicable to an electronic apparatus requiring operation at high temperature.
Russian Federation Patent No. 2052855 discloses as a cathode material of an alloy of iridium, lanthanum or cerium, tungsten, and rhenium. In this patent, the cathode lifetime has been increased by including in the alloy tungsten or rhenium, but the latter two metals are fragile, and thus the cathode including them is also fragile and the electron emission power decreases.
Further exemplars of the art are U.S. Pat. No. 5,519,280 issued to Shon et al for Oxide Cathode, U.S. Pat. No. 6,124,666 issued to Saito et al. for Electron Tube Cathode, U.S. Pat. No. 3,436,584 issued to Hughes et al. for Electron Emission Source with Sharply Defined Emitting Area, U.S. Pat. No. 5,982,083 issued to Ju et al. for Cathode for Electron Tube, U.S. Pat. No. 5,072,149 issued to Lee et al. for Cathode for Electron Gun and its Manufacturing Method, U.S. Pat. No. 5,580,291 issued to Redel et al. for Method for Manufacturing a Glow Cathode for an Electron Tube, U.S. Pat. No. 5,977,699 issued to Joo et al. for Cathode for Electron Tube, U.S. Pat. No. 5,808,404 issued to Koizumi et al. for Electron Tube Including a Cathode Having an Electron Emissive Material Layer, U.S. Pat. No. 5,828,165 issued to Clerc et al. for Thermionic Cathode for Electron Tubes and Method for the Manufacture Thereof, and W.O. Patent No. 00/21110 to Choi et al. for Cathode Material of Electron Beam Device and Preparation Method Thereof.
It is therefore an object to provide a cathode material with an improved lifetime and mechanical properties for an electron beam apparatus as well as excellent electron emission power.
It is another object to have a cathode material that has low reactivity with the components of the heater.
It is yet another object to have a cathode material used for an electron beam device that is not highly fragile and has higher adhesive strength.
It is still yet another object to have a cathode material that has an improved lifetime and increased emission power while not increasing production cost and still having ease of manufacture.
In order to achieve the above objectives, the invention provides a cathode material having between 0.5 to 9.0% by weight of a rare earth metal of the cerium group, between 0.5 to 15% by weight of tungsten or rhenium or both tungsten and rhenium, between 0.5-10% by weight of carbon and the remainder of iridium. When not mentioned explicitly, the percentage is based on the total weight of the cathode material.